The combined cycle power plant is a power generation system which has combined gas and steam turbine plants. In this system, the gas turbine takes charge of a higher-temperature range of the thermal energy, while the steam turbine takes charge of the remaining lower-temperature range, thereby efficiently recovering and using the thermal energy. This is currently the most common power generation system.
In order to improve the efficiency, the development of the combined cycle power plant has been focused on how high the higher-temperature range of the gas turbine can be set.
To realize a higher-temperature range, a cooling system must be provided in consideration of the heat resistance of the turbine structure. Conventionally, air is used as a cooling medium in such a cooling system.
However, as long as air is used as the cooling medium, even if a desirable higher-temperature range can be achieved, the plant will inevitably suffer from (i) loss of power necessary for boosting the air (which was used for cooling) up to a specified pressure by using an internal air compressor, and (ii) lowering of the average gas temperature and thus the energy of the gas because the air used for cooling target portions is finally made flow through the passage (in the turbine) together with the high-temperature gas. As a result of the above effects (i) and (ii), it is very difficult to further improve the thermal efficiency.
In order to solve the above problem and to further improve the efficiency, another type of combined cycle power plant has been proposed, in which the steam is used as the cooling medium of the gas turbine instead of air as mentioned above.
Japanese Unexamined Patent Application, First Publication, No. Hei 5-163960 discloses an example thereof. Although this publication discloses the concept that steam is used as a cooling medium for gas turbine 13, it also has many problems.
For example, in this system, when the gas turbine is activated or stopped, cooling steam is not supplied to the high-temperature portion to be cooled because it is considered in this system that at these times (i.e., when starting or stopping), the thermal load is relatively low and the cooling operation is unnecessary.
On the other hand, the compressed air of the gas-turbine compressor is the only cooling medium used as a substitute for the cool steam in such a power plant. Therefore, if the high-temperature portion of the gas turbine must be cooled when the gas turbine is activated or stopped, the compressed air of the gas-turbine compressor must be used.
In the above system disclosed in Hei 5-163960, when the compressed air of the gas-turbine compressor is used as a substitute, air in the gas turbine chamber is extracted to be introduced into the original steam cooling system. Therefore, this air passes through piping, valves, or the like, and thus a pressure loss inevitably occurs.
Here, the combustor is the highest-temperature part of all the high-temperature parts in the gas turbine, and the inner pressure thereof is only a little lower than the pressure of air in the gas turbine chamber. Therefore, if the above-described cooling with air is employed, the air pressure in the cooling path must be lower than the pressure in the combustion atmosphere of the combustor.
Accordingly, if a small hole such as a pinhole exists in the structure for cooling the combustor, high-pressure combustion gas will enter the cooling path via the hole, and the combustor and attached piping arrangement may be locally heated or burned.
It is an object of the present invention to solve the above problems in the conventional system and to maintain the safety of the system, and to provide a combined cycle power plant which can reliably operate for a long time.